The present invention relates to a sintered body having a hardened local portion and a method of manufacturing such a partially hardened sintered body.
Products having hardened local portions include rocker arms for moving intake and exhaust valves in internal combustion engines. The rocker arm has a sliding surface held in sliding contact with a cam or a valve, and the sliding surface is required to be resistant to abrasive wear. Rocker arms for internal combustion engines are typically in the form of steel forgings, iron-based sintered bodies, and aluminum die castings.
Steel forged rocker arms are sufficiently strong and rigid. However, a number of machining steps are required to grind the forged rocker arms and an abrasion-resistant member of cemented carbide needs to be brazed or otherwise bonded to the sliding surface to be held in contact with a valve or a cam.
The iron-based sintered rocker arms do not need to be machined to a large extent after the sintering process. However, an abrasion-resistant member of cemented carbide has to be brazed, in an inert atmosphere, to the sliding surface to be held in contact with a valve or a cam.
The aluminum die-case rocker arms have an abrasion-resistant member of cemented carbide or ceramics cast on the sliding surface to be held in contact with a valve or a cam. However, the abrasion-resistant member thus attached to the aluminum die-cast rocker arms is not necessarily satisfactory. Japanese Laid-Open Patent Publication No. 62(1987)-38810 discloses a rocker arm of aluminum alloy which has a sliding surface for contact with a cam, the sliding surface being in the form of a composite layer which comprises fine powder of an intermetallic compound or ceramic material dispersed in an aluminum alloy matrix. The sliding surface of the disclosed rocker arm, however, does not have a satisfactory level of abrasion resistance because it is difficult to produce a composite structure of the intermetallic compound or ceramic material in the aluminum alloy matrix. In addition, a high-density energy source such as a laser gun should be employed to construct the composite layer.